well ya its an exteme idea. But If mexico became part of the United States, it would cost billions more dollars and decades of reform to bring them up to date with us. Mexico is much poorer economically then us and much more corrupt.
Look at soviet-era germany as an example. West germany was doing great economically on its own, while east germany was poverty struck because of years of rule under the soviet union. After the fall of the berlin wall, when the wealthy west germany and poor east germany came together again, the poor, poverty ridden side dragged west germany's economy down a ton.

Personally i do kinda like the idea of mexico becoming part of the united states and it would probably work out well for us in the long run. But would we really be willing to put that much effort into fixing up mexico after it became part of our country?